For the conductive material for the connecting part such as the connector terminal, bus bar, and so forth, used in electrical wiring for automobiles, consumer equipment, and the like, use is made of Cu or a Cu-alloy, with Sn plating applied thereto, (including an Sn-alloy plating such as solder plating and so forth) except the case of an important electrical circuit requiring high reliability of electrical connection, against a low-level signal voltage and current. Sn plating has been in widespread use because it is lower in cost in comparison with Au plating, and any other means for surface treatment. Among others, Sn plating containing no Pb from a standpoint of coping with recent regulations against material causing environmental impacts, and particularly, reflow Sn coating, and hot dip Sn coating, on which there have hardly been reported a case of short circuit trouble due to occurrence of whiskers, are now in the mainstream.
As a leap forward development has recently been made in electronics, rapid progress has been seen in higher use of electrical equipment in, for example, automobiles, in an attempt to pursue safety, environmental friendliness, and driving comfort. As a result, there occurs an increase in the number of circuits, weight thereof, and so forth, leading to an increase in space occupied, and energy consumption, so that there arise requirements for a conductive material for a connecting part capable of providing a satisfactory performance required of the connecting part such as a connector terminal and so forth even in the case of a multi-way connector, further reduction in size as well as weight, and a connecting part mounted in an engine room.
The Sn plating is applied to the conductive material for the connecting part mainly for the purpose of providing a surface thereof with corrosion resistance while obtaining a low contact resistance at electrical contacts and junctions and securing solderability when the conductive materials for the connecting parts are joined together by soldering. An Sn covering layer is a very soft conductive film, and an oxidized surface film thereof is prone to fracture. Accordingly, in the case of a fitting type terminal made up of a male form terminal in combination with a female form terminal, electrical contacts, such as indents, ribs, and so forth, tend to easily form gastight contact due to adhesion occurring between the plating layers to be thereby rendered suitable for obtaining a low contact resistance. Further, in order to maintain the low contact resistance in applications, an Sn plating layer is preferably larger in thickness, and it is important to increase a contact pressure at which the electrical contacts are pressed against each other.
However, if the Sn plating layer is rendered larger in thickness, and the contact pressure at which the electrical contacts are pressed against each other is increased, this will cause an increase in a contact area between the Sn covering layers, and an increase in an adhesion force therebetween, so that there occurs an increase in a deformation resistance due to the Sn plating layer being turned up at the time of insertion of the terminal, and an increase in a shearing resistance for shearing adhesion, thereby resulting in an increase in an insertion force. A fitting type connecting part large in insertion force will cause poor efficiency of assembling work, and deterioration in electrical connection due to wrong fitting. Accordingly, there is a demand for terminals low in insertion force so that the total insertion force thereof does not become greater than that in the past even if the number of poles is increased.
Further, in the case of a small-sized Sn plated terminal, and so forth, with a reduced contact, pressure under which electrical contacts are pressed against each other, for the purpose of reducing the insertion force thereof, and wear occurring thereto at the time of insertion of the terminal, and pull-out thereof, not only it becomes difficult to maintain a low contact resistance in subsequent applications but also the electrical contacts are caused to undergo slight sliding due to vibration, thermal expansion/contraction, and so forth, during applications, so that the small-sized Sn plating terminal will be susceptible to occurrence of a slight-sliding wear phenomenon causing an abnormal increase in contact resistance. It is presumed that the slight-sliding wear phenomenon is induced by wear occurring to the Sn covering layers at electrical contacts, due to the slight-sliding, and by deposition of a large amount of resultant Sn oxide between the electrical contacts, due to repetition of the slight-sliding. For reasons described as above, there is a demand for a terminal low in the insertion force, excellent in resistance to wear upon insertion thereof, and pull-out thereof as well as resistance to wear due to the slight-sliding so as to be capable of maintaining a low contact resistance in spite of an increase in the number of actions for the insertion and pull-out, and the slight-sliding occurring to the Sn plating layers at electrical contacts.
In the following Patent Documents 1 to 6, respectively, there is described material for a fitting type terminal, wherein an Ni plating layer as an undercoat is formed as necessary on the surface of a base material composed of Cu or a Cu-alloy, and after forming a Cu plating layer, and an Sn plating layer in that order on the top of the Ni plating layer, a reflow process is applied thereto, thereby forming a Cu—Sn alloy covering layer composed primarily of Cu6Sn5 phase. According to description in those Patent Documents, the Cu—Sn alloy covering layer formed by the reflow process is harder as compared with the Ni plating layer, and the Cu plating layer, and owing to presence of the Cu—Sn alloy covering layer as an undercoat layer of the Sn covering layer remaining on the uppermost surface of the material, it is possible to decrease the insertion force of the terminal. Further, a low contact resistance can be maintained by the agency of the Sn covering layer present on the uppermost surface.
Furthermore, in the following Patent Documents 7 to 9, respectively, there is described material for a fitting type terminal, wherein a Cu plating layer as an undercoat is formed as necessary on the surface of a base material composed of Cu or a Cu-alloy, and after forming an Sn plating layer on the top of the Cu plating layer, a reflow process is applied thereto as necessary before heat treatment, thereby forming an intermetallic compound layer composed primarily of Cu—Sn, and an oxidized film layer as necessary in that order. According to description in those Patent Documents, a Cu—Sn alloy covering layer is formed on the surface of the material by the heat treatment, thereby enabling the insertion force of the terminal to be further decreased.    Patent Document 1: JP-A No. 68026/2004    Patent Document 2: JP-A No. 151668/2003    Patent Document 3: JP-A No. 298963/2002    Patent Document 4: JP-A No. 226982/2002    Patent Document 5: JP-A No. 135226/1999    Patent Document 6: JP-A No. 60666/1998    Patent Document 7: JP-A No. 226645/2000    Patent Document 8: JP-A No. 212720/2000    Patent Document 9: JP-A No. 25562/1998